(1) Field of the Invention
The present invention relates to a photosensitive polymer composition. More specifically, it relates to a water- or alcohol-developable photosensitive polymer composition having an excellent ink-transfer property and solvent resistance, especially suitable for use in the formation or production of a flexographic printing plate.
(2) Description of the Prior Art
Heretofore, rubber printing plates for flexographic printing have generally been produced as follows. That is, original plates are first prepared by the etching of metal plates. Then, matrixes are prepared from the original plates by heat pressing a thermosetting resin (e.g., a phenol resin) to the original plates and, thereafter, rubber is poured or casted into the matrixes, followed by pressing to form the rubber printing plates. This method, however, has disadvantages in that the skilled operation, high costs and a lot of time are required for preparing the rubber printing plates and also in that the image-reproducibility of the resultant rubber printing plates per se is poor.
In order to eliminate the above-mentioned disadvantages, various attempts have recently been made to directly prepare flexographic printing plates from photosensitive elastomer compositions. That is, the printing plates are prepared from the photosensitive elastomer compositions as follows. A negative or positive image-bearing film having transparent portions is placed closely against a photosensitive layer comprising an elastomer and a photopolymerizable vinyl monomer. The photosensitive layer is then irradiated with activating light through the image-bearing film. Thus, the portions of the photosensitive layer corresponding to the transparent portions of the image-bearing film are photopolymerized. The unpolymerized portions of the photosensitive layer are removed with an appropriate solvent to form the desired relief image.
Various elastomers usable in the formation of photosensitive flexographic printing plates utilizing a photopolymerization reaction have been proposed. For instance, U.S. Pat. Nos. 2,948,611 and 3,024,180 disclose photosensitive polymer compositions suitable for use in flexographic printing comprising polychloroprene rubber or polyurethane rubber and a photopolymerizable monomer having a terminal ethylenically unsaturated bond incorporated therein. U.S. Pat. No. 3,674,486 also discloses a photosensitive polymer system comprising styrene-butadiene rubber or styrene-isobutylene rubber and a photopolymerizable monomer having a terminal unsaturated bond incorporated therein. Furthermore, French Pat. No. 2103825 discloses a photosensitive polymer system containing, as a main polymer, acrylonitrile-butadiene rubber or styrene-isobutylene rubber, and Japanese Examined Patent Publication (Kokoku) No. 52-28044 discloses a photosensitive polymer system containing, as a main polymer, 1,2-polybutadiene.
However, these printing plates have disadvantages in that the rubber elasticity and ink-transfer property thereof are poor as compared with rubber printing plates. Also, the solvent resistance, abrasion resistance, and durability thereof are poor, as compared with the conventional vulcanized rubber printing plate. Furthermore, it is pointed out that the greatest disadvantage of the above-proposed printing plates is that the working atmosphere is remarkably polluted by the use of a halogenated hydrocarbon such as 1,1,1-trichloroethane as a developing solvent in the developing process to dissolve and remove the unpolymerized portions of the printing plate.